Water-vapor plasma torch, and wear-detection and process-control method to be used with such a water-vapor plasma torch

ABSTRACT

The invention relates to a water-vapor plasma torch ( 7 ) for cutting a workpiece ( 21 ), comprising a feed line ( 8 ) for a liquid ( 9 ), a heating device ( 22 ), and an evaporator ( 23 ) for forming a gas ( 20 ) from the liquid ( 9 ), a cathode ( 24 ) detachably connected to a movably mounted piston rod ( 25 ), and a nozzle ( 26 ) with an outlet opening ( 27 ) for the gas ( 20 ), as well as to a wear-detection and process-control method to be used with such a water-vapor plasma torch ( 7 ). To create such a water-vapor plasma torch ( 7 ) including wearing-part detection, at least one temperature sensor ( 28 ) is arranged within the piston rod ( 25 ), said temperature sensor being connected to a control unit ( 4 ), so that a wear of the cathode ( 24 ) and the nozzle ( 26 ) can be concluded from the temperature values detected, and that the control of the water-vapor plasma cutting process is influenceable.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and Applicant claims priority under35 U.S.C. §§120 and 121 of parent U.S. patent application Ser. No.12/310,410 filed Feb. 24, 2009, which application is a national stageapplication under 35 U.S.C. §371 of PCT Application No.PCT/AT2007/000438 filed on Sep. 14, 2007, which claims priority under 35U.S.C. §119 from Austrian Patent Application No. A 1546/2006 filed onSep. 15, 2006, the disclosures of each of which are hereby incorporatedby reference. A certified copy of priority Austrian Patent ApplicationNo. A 1546/2006 is contained in parent U.S. application Ser. No.12/310,410. The International Application under PCT article 21(2) wasnot published in English.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a water-vapor plasma torch for cutting aworkpiece, comprising a feed line for a liquid, a heating device, and anevaporator for forming a gas from the liquid, a cathode detachablyconnected to a movably mounted piston rod, and a nozzle with an outletopening for the gas.

The invention further relates to a method of detecting the wear of thecathode of a water-vapor plasma torch during and after a cuttingprocess, with said cathode being detachably connected with a piston rod,wherein the heat generated by an electric arc is thermally coupled intothe piston rod connected with the cathode so that when the temperaturechanges at the cathode, the temperature within the piston rod will alsochange.

Moreover, the invention relates to a method of detecting the wear of theanode of a water-vapor plasma torch, said anode being designed as anozzle with an outlet opening, wherein a gas escapes through the outletopening formed by evaporation of a liquid, fed via a feed line, by meansof a heating device, and an evaporator, wherein the nozzle is worn outby enlarging the outlet opening, thus increasing the flow rate of theliquid.

Likewise, the invention relates to a method of controlling a cuttingprocess conducted by means of a water-vapor plasma torch, wherein aliquid is fed to the torch via a feed line, and wherein the liquid isevaporated into a gas by means of a controlled heating device and anevaporator.

2. The Prior Art

From the prior art, in particular from EP 1 522 371 A1, it is known toarrange a plurality of sensors in a plasma torch which detects thetemperature or voltage, e.g. Here, the individual sensors are connectedto a processor in the plasma torch which, in turn, is connected to theprocessor in the current source via a data line. Thus, the cuttingprocess can be controlled based on the measurement values provided bythe sensors.

Here, it is disadvantageous that the size of the plasma torch increaseswith each sensor installed therein, and with the integration of theprocessor and the electronics associated therewith. This makes thehandling of the plasma torch at least partially more difficult.Likewise, the risk of interference is increased by using sensitiveelectronics in the plasma torch. Furthermore, no technical solution forwear detection is described in EP 1 522 371 A1.

Moreover, it is known from the prior art to control the temperature ofan evaporator, which evaporates liquid, in a torch of a water-vaporcutting device.

Here, it is disadvantageous that the heat introduced into a cathode viathe electric arc may influence the temperature control of the evaporatorin an unpredictable manner since the heat introduced by the cathode isnot taken into consideration when controlling the temperature of theevaporator. Thus, the torch may possibly be overheated and destroyed.

SUMMARY OF THE INVENTION

The object of the invention is to create a water-vapor plasma torchwhich allows for an automatic wear detection, in. particular of acathode and an anode, as well as for process-control improvement, inparticular control for evaporating the liquid operational medium.

Further objects of the invention reside in creating a wear-detectionmethod to be used with a water-vapor plasma torch, as well as in aprocess-control method to be used with such a water-vapor plasma torch.

The first object of the invention is achieved in that at least onetemperature sensor is arranged within the piston rod, said at least onetemperature sensor being connected to a control unit so that a wear ofthe cathode and the nozzle can be concluded from the temperature valuesdetected, and that the control of a water-vapor plasma cutting processis influenceable. The temperature detected by at least one of thetemperature sensors arranged within the piston rod enable the controlunit to evaluate the temperature changes, and the cathode wear of thewater-vapor plasma torch can be concluded from the temperature changes.

If a flow-rate sensor is additionally provided for measuring the flowrate of the liquid in the feed line, the nozzle wear can be concludedfrom the flow-rate change. Moreover, the heating output of the heatingdevice can be detected, and the nozzle wear of the torch can beconcluded from the heating output adapted to the flow-rate through thenozzle.

What is essential here is that the function of the water-vapor plasmatorch is not affected by the at least one temperature sensor. Theintegration of the temperature sensor does not increase the structuralsize of the water-vapor plasma torch either, thus not negativelyinfluencing handling thereof. Thanks to the wear detection the controlunit is substantially permanently informed about the state of thewearing parts, i.e. cathode and nozzle, thus allowing for the wearingparts to be used until they have been completely worn out. Of course, aconstant quality of the cutting process is safeguarded here. The wearcan be correspondingly displayed to the user of the water-vapor plasmatorch so that a change of the worn-out parts will be recognized in time.This substantially minimizes the risk of the water-vapor plasma torchbeing destroyed, e.g. by overheating, thus substantially prolonging theservice life of the torch. Moreover, thanks to the early wear detectionit is achieved in an advantageous manner that the stability and thequality of the cutting process will not be affected.

The temperature detection of the piston rod allows for the heat balancein the water-vapor plasma torch to be controlled in a substantiallybetter and faster manner so that the liquid operational medium will becompletely evaporated already at minimum required heating output. Thus,an overheat of the water-vapor plasma torch can be substantiallyexcluded, whereby the service life of the torch is substantiallyprolonged. Accordingly, the temperature of the torch is monitored by theinventive temperature sensor so that the control unit can interrupt thecutting process when a certain temperature threshold is reached.

The features of an embodiment of the invention ensure a precisedetection of the piston-rod temperature.

By the measures of another embodiment it is advantageously achieved thatthe function of the piston rod, in particular its movement, will not berestricted.

According to another embodiment, it is achieved in an advantageousmanner that the heat balance of the water-vapor plasma torch can becontrolled in a precise manner.

The measure of another embodiment is also of advantage since it allowsfor a stable electric arc to be reached for the cutting process.

By the measure of another embodiment it is advantageously achieved thatthe point of time when the wearing parts are to be exchanged isdisplayed to the user in a visual manner.

The object of the invention is also achieved by an above-mentionedmethod of detecting the wear of the cathode of the water-vapor plasmatorch, wherein the temperature within the piston rod is detected by atleast one temperature sensor arranged within the piston rod, and whereinthe temperature change is evaluated by a control unit connected with thetemperature sensor, and the temperature change will be consulted as ameasure for the wear of the cathode. Here, it is advantageous that thenecessary pre-heating time of the torch can be precisely adapted to therespective ambient conditions and be thus kept as short as possible.Likewise, it is advantageous that the dynamic behavior of the torch canthus be substantially improved since the information on a temperaturechange is available to the control unit at an earlier point of time viathe temperature sensor of the cathode.

The object of the invention is also achieved by an above-mentionedmethod of detecting the cathode wear, wherein the temperature within thepiston rod is detected by at least one temperature sensor arrangedwithin the piston rod, and wherein the temperature change is evaluatedby a control unit connected with the temperature sensor, and thetemperature change will be consulted as a measure for the wear of thecathode.

The inventive object is also achieved by an above-mentioned method ofdetecting the nozzle wear of a water-vapor plasma torch, wherein theflow rate of the liquid is measured in the feed line, and wherein thechange in the flow rate is consulted as a measure for the wear of thenozzle.

Likewise, the output of the heating element can be detected, and theheating output which is a function of the flow rate of the liquid can beconsulted when assessing wear of the nozzle.

The inventive object is also achieved by an above-mentioned controlmethod of a cutting process conducted by means of a water-vapor plasmatorch, wherein the temperature within a piston rod of the torch isdetected by at least one temperature sensor arranged within the pistonrod, and wherein the heating device is controlled as a function of thetemperature within the piston rod in a manner to ensure the completeevaporation of the liquid fed via the feed line.

The measures of another embodiment allow for the heating device to becontrolled only in case of a properly turned-on torch so that nolife-threatening voltage can apply between the cathode and the nozzleand/or between the cathode and the workpiece when the torch isdisassembled.

Further advantages of the invention can be learned from the description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be explained in more detail by way of theenclosed schematic drawings. Therein:

FIG. 1 shows an exemplary representation of a water-vapor cuttingdevice;

FIG. 2 shows a cross-section through an embodiment of a water-vaporplasma torch;

FIG. 3 shows a cross-section through a water-vapor plasma torch whichincludes an inventive temperature sensor within the piston rod; and

FIG. 4 shows an example of the temperature time course at the piston rodfor detecting the cathode wear from the temperature change aftercompletion of the welding process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Initially, it is pointed out that same parts of the exemplary embodimentare designated by same reference numbers.

FIG. 1 shows a water-vapor cutting device 1 with a basic unit 2 for awater-vapor cutting process. The basic unit 2 comprises a current source3, a control unit 4, and a blocking element 5 assigned to the controlunit 4. The blocking element 5 is connected to a reservoir 6 and awater-vapor plasma torch 7 via a feed line 8 so that the water-vaporplasma torch 7 can be supplied with a liquid medium or liquid 9 presentin the reservoir 6. The water-vapor plasma torch 7 is supplied withelectric energy via lines 10, 11 which are connected to the currentsource 3.

To provide cooling of the water-vapor plasma torch 7, the same isconnected to a liquid reservoir 14 via a cooling circuit 12, with a flowcontrol 13 being possibly interposed. When the torch 7 or thewater-vapor cutting device 1 is put into operation, the control unit 4can initiate the cooling circuit 12, thus achieving a cooling of thetorch 7 via the cooling circuit 12. To create a cooling circuit 12, thetorch 7 is connected to the liquid reservoir 14 via cooling lines 15,16.

Furthermore, the water-vapor cutting device 1 may include an inputand/or output unit 17 via which the most different parameters andoperational modes of the water-vapor cutting device 1 can be set anddisplayed. The parameters set via the input and/or output unit 17 areforwarded to the control unit 4 which appropriately activates theindividual components of the water-vapor cutting device 1.

Moreover, the water-vapor plasma torch 7 may include at least oneoperational element 18, in particular a pushbutton 19. Via theoperational element 18, in particular via the pushbutton 19, the usercan inform the control unit 4 of the torch 7 that a water-vapor cuttingprocess shall be started and/or conducted by activating or deactivatingthe pushbutton 19. Furthermore, e.g. presettings can be adjusted via theinput and/or output unit 17, in particular as regards the material to becut, the liquids used, and, for example, the characteristic curves ofthe current and the voltage can be predefined, e.g. Of course, furtheroperational elements can be provided on the torch 7 via which one orseveral operational parameters of the water-vapor cutting device 1 areset. To this end, these operational elements can be connected to thewater-vapor cutting device 1, in particular to its control unit 4,directly via lines or a bus system.

After the pushbutton 19 has been actuated, the control unit 4 activatesthe individual components necessary for the water-vapor cutting process.For example, a pump (not illustrated), the blocking element 5, and thecurrent source 3 are activated first, thus introducing a supply of thetorch 7 with the liquid 9 via the feed line 8, as well as theelectric-energy supply. Subsequently, the control unit 4 will activatethe cooling circuit 12 so as to allow for a cooling of the torch 7. Ofcourse, the supply of the torch 7 with the liquid 9 can also be ensuredby the cooling of the torch 7 since the liquid 9 is heated to thetemperature necessary for the cutting process only in the torch 7. As alogical consequence, the cooling circuit 12, which comprises the flowcontrol 13, the liquid reservoir 14 and the cooling lines 15, 16, couldbe omitted. By the measure of supplying the torch 7 with the liquid 9and with electric energy, the liquid 9 is converted into a gas 20, inparticular plasma, in the torch 7 now by employing appropriately hightemperatures. A cutting process can be conducted on a workpiece 21 byusing the gas 20 escaping from the torch 7. Here, the liquid 9 isconverted into the gas 20 at least by the aid of a heating device 22preferably comprised of an appropriate heating element, and by the aidof an evaporator 23 preferably integrated into the torch 7.

An electric arc 33 is additionally required for a cutting process on aworkpiece 21 with a torch 7, illustrated in detail in FIG. 2. Theelectric arc 33 is ignited by the control unit 4 and by actuating thepushbutton 19, and it burns between a cathode 24, which is connected tothe piston rod 25, thus being integrated in the torch 7, and whichpreferably is connected with the negative pole of the current source 3,and an anode, which is formed by a nozzle 27 and connected with thepositive pole of the current source 3. When the torch 7 approximates tothe workpiece 21, the positive pole of the current source 3 is switchedoff of the nozzle 26, whereby the electric arc 33 will correspondinglybe forced outwards by the gas 20 through the outlet opening 27 in thenozzle 26, thus burning between the cathode 24 and the workpiece 21. Tothis end, the workpiece 21 is connected with the positive pole of thecurrent source 3. That is, when the electric arc 33 is burning betweenthe cathode 24 and the workpiece 21, the control unit 4 increases thecurrent appropriately, finally producing a plasma jet of an energydensity which is high enough for melting-on, which is why it can be usedfor cutting workpieces 21.

When separating the workpiece 21 by the aid of a water-vapor plasmatorch 7, many components, in particular the cathode 24 and the nozzle26, are subjected to very high temperatures, and to a very high current,thus exhibiting great wear. The quality of the cutting process stronglydepends on the degree of wear of the wearing parts. Consequently, a weardetection integrated into the torch 7 is advantageous for qualitycontrol.

According to the invention, the wear is detected via at least onetemperature sensor 28 integrated into the piston rod 25. The temperaturesensor 28 transmits the temperature detected preferably to the controlunit 4 which is capable of drawing conclusions as to the wear of thewearing parts from the temperature changes. Such a wear detection alsoallows for the torch 7 to be prevented from thermal overload.

In FIG. 3, a torch 7 is shown which includes the inventive temperaturesensor 28 within the piston rod 25. The piston rod 25, and thus also thecathode 24, are movably mounted within the torch 7. This is why it isnecessary for the temperature sensor 28 to be arranged within thesubstantially cylindrical piston rod 25 such that it will not restrictmovement of the piston rod 25, and of the cathode 24 connectedtherewith. For this reason, the temperature sensor 28 is preferablyrod-shaped. Preferably, the temperature sensor 28 is comprised of asemiconductor resistance, for example a PTC(positive-temperature-coefficient) resistance, e.g. a PT100. Likewise,it is also possible to select a resistance with a negative temperaturecoefficient, a so-called NTC (negative-temperature-coefficient)resistance. Certainly, the temperature sensor 28 may also be comprisedof a thermal element or the like.

For receiving the temperature sensor 28 within the piston rod 25, thelatter has a recess 29 or bore whose form corresponds to that of thetemperature sensor 28. The recess 29 is provided in the edge region ofthe piston rod 25 so that the space for the elements necessary formovement of the piston rod 25 (e.g. a spring) will not be limited.Furthermore, the recess 29 extends preferably in parallel to thelongitudinal axis of the piston rod 25, and substantially as far as tothe cathode 24. Preferably, the temperature sensor 28 is clamped, orpressed, into the recess 29 and, optionally, glued thereto.

To allow the temperature sensor 28 to transfer the temperature detectedto the control unit 4, the temperature sensor 28 comprises electricconnections 30. In the region of the piston-rod mounting, theseconnections 30 are fed outwards through a passage 31 opposite thecathode 24, and can be connected to the control unit 4. Of course, thepassage 31 is no hindrance for the movably mounted piston rod 25.

According to the invention, the wear of the cathode 24 and the nozzle 26can now be detected with a such-arranged temperature sensor 28 only.

The wear detection of the cathode 24 can be effected as follows.

During a cutting process, the electric arc 33 necessary therefore isburning between the cathode 24 and the workpiece 21. Preferably, theelectric arc 33 forms in the center of the contact surface of thecathode 24, wherein, at this site, a pin or bolt 32, e.g. made ofhafnium, is pressed into the cathode 24. The electron-emittingproperties of hafnium, or, for example, also of zirconium or the like,cause the electric arc 33 to apply directly to the bolt 32. This ensuresa stable electric arc 33 for the cutting process. Yet, the bolt 32 isincreasingly worn out when the torch 7 or water-vapor cutting device 1is in continuous operation, and a deeper weld penetration is caused inthe bolt 32 by the electric arc 33, thus coupling more power or heatinto the cathode 24. Since the cathode 24 is thermally connected withthe piston rod 25, i.e., for example, by means of a screw connection, acorresponding temperature increase results within the piston rod 25.This temperature increase, e.g. of from about 160° C. to about 180° C.,is detected via the temperature sensor 28 and the control unit 4. Thecorresponding reference data and reference values for the temperatureincrease are deposited in the control unit 4 so as to allow for the wearof the cathode 24 to be concluded from the temperature increase. Sincethe temperature detected by the temperature sensor 28 depends at leastpartially on the position of the temperature sensor 28, two or moretemperature sensors 28, for example, may be arranged within the pistonrod 25. The control unit 4 may generate, e.g., a mean value from alltemperature values detected, enabling a precise conclusion as to thewear of the cathode 24.

The reference data in the control unit 4 may also include, e.g.,information on the period of time during which the temperature increasesas a function of the set current strength. If the temperature increasesquickly in a relatively short period of time, this means a strong wearof the cathode 4. This is the case, e.g., when the temperature increasesby 7° C. to 10° C. at a cutting period of 60 min.

Likewise, a temperature threshold can be deposited as a reference valuein the control unit 4, wherein the cutting process will be stopped forthe sake of the torch 7 when said threshold has been exceeded so as toallow for the torch 7 to be cooled by means of the liquid 9. Thus, thecontrol unit 4 may take the appropriate measures for safeguarding thequality of the cutting process as well as for preventing the torch 7from overheat. For example, the current for the cutting process may bereduced, the wear may be correspondingly displayed on the input and/oroutput unit 17 at the torch 7, indicating a necessary cathode change tothe user, or the cutting process may be stopped.

A further possibility of detecting a worn-out cathode 24 is to evaluatethe temperature increase of the piston rod 25 after a cutting process.According to the invention, this is achieved via the temperature sensor28 provided within the piston rod 25.

During a cutting operation there is a heat gradient from the tip of thecathode 24 to the opposite end of the piston .rod 25. This is the casesince the cathode 24 is heated up by the electric arc 33, with thepiston rod 25 being cooled by means of the liquid 9. Accordingly, thecathode 24 will no longer be heated up after a cutting process, and thepiston rod 25 will no longer be cooled, thus leveling off or adaptingthe heat gradient. Thus, after a cutting process, the temperature of thepiston rod 25 increases by the heat stored in the cathode 24. This canbe detected by a temperature sensor 28, and be evaluated by the controlunit 4, wherein the temperature increase may also be evaluated as afunction of time. This allows for the wear of the cathode 24 to beinventively concluded.

Such an evaluation is particularly done after short cutting processes,lasting less than 10 sec, e.g. This is required since the temperaturerelations between the evaporator 23, the cathode 24, and the piston rod25, will adjust after a certain period of time only, e.g. after about 10sec, so that the wear can be efficiently detected during a cuttingoperation only from this point of time on, as described above. It isapparent from the time course of the temperature 34 at the piston rod 25according to FIG. 4 that the wear can be detected efficiently only frompoint of time 35 on. This is why the temperatures in the water-vaporplasma torch 7 are generally higher in case of a short cutting processso that the reference values of the wear detection during a cuttingoperation, within a period of about the first ten seconds of the cuttingprocess, are too low to detect the wear and/or the control unit 4 wouldstop the cutting process due to overheat. These increased temperaturesare to be attributed to the fact that the water-vapor plasma torch 7 hasa temperature, e.g., of about 190° C. during idling or standby. Whenpushing the pushbutton 19 at point of time 36, starting the cuttingprocess, the piston rod 25 will be cooled, and the cathode 24 will beheated up by the electric arc. Only if the cutting process is stoppedafter about 10 seconds at point of time 37, the piston rod 25 will havea temperature, e.g., of about 180° C., and the cathode 24 will have beenheated up to 300° C. As already known, the heating-up of the cathode 24depends on its wear, and the heat stored in the cathode 24 will becoupled into the piston rod 25 after switching-off. Due to thetemperature difference of about 120° C. the piston rod 25 may now beheated up to 200° C., at least for a short term, as can be seen in thetemperature course 38, whereupon the idling temperature, e.g. of about190° C., will again adjust. Such a temperature behavior is known as aso-called “temperature overshoot”. When a threshold value 39 is definedin this temperature region and deposited in the control unit 4, a wearof the cathode 24 can be detected only in case the threshold value 39has been exceeded once or several times. When the temperature sensor 28measures that the threshold value 39 has been exceeded—as can be seen atpoint of time 40, e.g.—this serves as a measure for a cathode 24 whichis at least starting to wear out. This is why several successiveexceedances, i.e. one exceedance with each short cutting process, ispreferably necessary for activating wear-displaying.

Preferably, in addition to monitoring the threshold value 39, the slopeof the temperature increase, i.e. the change of the temperature as afunction of time, is evaluated. To this end, a timer is started, e.g.when the cutting process is stopped, which has only an effect on weardetection when the threshold value 39 has been exceeded. In this case,the time required for reaching the threshold value 39 will be taken intoconsideration when detecting wear. When the threshold value 39 has beenexceeded very quickly, this is taken as an indication for an alreadyvery strong worn-out cathode 24, whereas when the threshold value 39 hasbeen exceeded more slowly, this is taken as an indication for an onlyslightly worn-out cathode 24. The corresponding comparative values maybe deposited in the control unit 4.

The threshold value 39, and the comparative values, may be appropriatelyadapted to the dimensions of the cathode 24 and/or the piston rod 25,i.e. the power class of the water-vapor plasma torch 7.

The combination of wear detections during a cutting process and after ashort cutting process allows for the user to provide information on thestate of the cathode 24, preferably via the wear display, at any time.

Wear detection of the nozzle 26 may be effected as follows.

The outlet opening 27 of the nozzle 26 becomes more and more worn-outwhen the number of cutting processes increases, whereby the diameter ofthe outlet opening 27 is widened and/or a weld puddle is formed aroundthe outlet opening 27, shortening the channel length of the same. This,in turn, means that the amount of the gas 20 escaping from the outletopening 26 will be correspondingly increased. This results in thatcorrespondingly more liquid 9 will be fed to the torch 7, which liquidhas to be evaporated in the same period of time. A correspondinglyhigher heating output of the heating device 22 is necessary forevaporating a larger amount of liquid 9. According to the invention, theheating output is controlled as a function of the temperature detectedby the temperature sensor 28, as will be described in more detail below.The heating output is directly proportional to the diameter of theoutlet opening 27 of the nozzle 26. Thus, the control unit 4 can drawconclusions as to the wear of the nozzle 26 from the heating output, andtake appropriate measures, as described above.

A further possibility of detecting the wear of the nozzle 26 is tointegrate a flow-rate sensor (not illustrated) into the feed line 8 ofthe liquid 9. This flow-rate sensor provides corresponding data to thecontrol unit 4 which can draw conclusions as to the wear of the nozzle26 from the increase in the flow rate. As known, the wear of the nozzle26 depends, however, on many factors, e.g. heating output, currentstrength, nozzle diameter, gas flow, etc., which factors the controlunit 4 has to appropriately take into consideration when drawingconclusions as to the wear. Preferably, a nozzle 26 is regarded to beworn-out when the flow rate is higher than the nominal flow rate, or thenominal value for the flow rate, by about 60%. This is the case, e.g. at15 ml/min.

Certainly, it is also possible to combine the wear detection via aflow-rate sensor with the wear detection via heating output, allowingfor even more precise conclusions to be drawn as to the wear of thenozzle 26.

According to the invention, a temperature sensor 28 integrated into thepiston rod 25 may also be used for improving the heating-output controlsystem.

As known from the prior art, the liquid 9 serves for cooling the cathode24 and/or the piston rod 25, thus prolonging their service life, on theone hand, and, in the vaporous state, the liquid 9 serves as a startingmedium for the cutting process, on the other hand. At first, the liquid9 circulates around the piston rod 25, and will thereafter be broughtinto the vaporous state in the evaporator 23 by supplying the heatingdevice 22 with current.

To achieve a qualitatively high and stable cutting process, attentionhas to be paid that the liquid 9 is evaporated completely withoutoverheating the torch 7. The inventive temperature sensor 28 within thepiston rod 25 enables a precise control of the heating output of theheating device 22. This is of particular importance when a cuttingprocess is being started with a completely cool torch 7 since in thiscase the risk of overheating the torch 7 or of a torch 7 withinsufficient temperature is especially high. For conducting a cuttingprocess, the evaporator 23 is heated up to a temperature of about 190°C. so as to allow for the liquid 9 to be completely evaporated. Here, itis a prerequisite that the piston rod 25, which is heated up by theelectric arc 33 via the cathode 24, already pre-heats the liquid 9, asis the case during normal operation. When staring the cutting operationwith a “cold” torch 7 and/or a “cold” piston rod 25, no pre-heating isnecessary. Thus, the evaporator 23 would have to be heated up to asubstantially higher temperature, increasing thermal load on the torch7.

The risk that a torch 7 and/or an evaporator 23 has (have) beenpreheated insufficiently, causing insufficient temperature in the torch7, is even more likely. Here, the evaporator 23 is provided with toocold a liquid 9 since the latter has not been pre-heated by the coldpiston rod 25. To put it differently, the temperature of the evaporator23 is too low since the latter has not received any information on thetemperature of the piston rod 25 but which information is essential forpre-heating the liquid 9. Consequently, the temperature of theevaporator 23 is too low to evaporate the cold liquid 9 completely. Thisis why the liquid 9 accumulates in the region of the outlet opening 27and/or escapes therefrom, having negative effects on the service life ofthe torch 7, and the wearing parts.

Accordingly, the inventive control, in particular for the startingprocedure of the cutting process, is effected such that the temperatureof the piston rod 25 detected by the temperature sensor 28 istransferred to the control unit 4. As a function of this temperature,the control unit 4 can detect how much the fed liquid 9 has already beenpre-heated by the temperature of the piston rod 25. This results in theheating output necessary for the heating device 22 without putting thetorch 7 at the risk of overheating. In case of too high a temperature ofthe piston rod 25, the heating device 22 is supplied with less power. Incorrespondence therewith, the heating device 22 is supplied with morepower in case of too low a temperature of the piston rod 25.Additionally, more heat may be introduced into the piston rod 25 byselectively increasing the cutting current for a short time (e.g. by 10A), whereby the heating output is lower, thus allowing for a stable andcontinuous cutting process to be ensured. In addition, the flow rate orthe pressure (e.g. by 1 bar) of the liquid 9 may be reduced for a shorttime, whereby less liquid 9 has to be evaporated, and, thus, a lowerheating output is necessary.

Moreover, by means of the temperature sensor 28, it is also achievedthat the torch 7 and/or the piston rod 25 always have the temperaturenecessary to appropriately heat up the liquid 9 for a stable cuttingprocess. According to this, also a “cool” torch 7 can be detected whichis of particular importance for the start of a cutting process.

As already mentioned, such a control is particularly important duringthe starting stages of the cutting process since in these cases a fastevaporation of the liquid 9 is necessary to safeguard a stable cuttingprocess. The inventive control allows for the a stable cutting processto be ensured in each process stage by means of the at least onetemperature sensor 28.

That is, the inventive temperature sensor 28 ensures the necessary,complete evaporation of the liquid 9 in each case so as to provide for acutting process of optimum quality. A power element (not illustrated)integrated into the water-vapor cutting device supplies the currentnecessary for forming the electric arc 33. A so-called protective-capmonitoring means can be integrated into the torch 7 so as to allow forthe electric arc 33 to be ignited in the assembled state of the torch 7only for the sake of safety. Here, the power element can only bede-blocked and/or the cutting process can only be started when ashort-circuit has been detected between the cathode 24 and the nozzle26. Such a short-circuit detection may be effected by means of ahigh-ohmic, low-volt voltage source.

Of course, the inventive wear detection, and the process control, mayalso include a separate control realized in a microcontroller or thelike, e.g. Yet, the control is preferably effected centrally via thecontrol unit 4. Since the wear, in particular of the cathode 24 and thenozzle 26, depends on other factors, e.g. heating output, currentstrength, nozzle diameter, gas flow, and the like, the control unit 4may also take these factors into consideration when detecting the wear.

Here, a calibration is of advantage. In particular, tolerance limits,e.g. of the temperature sensor 28, or interpolations in the referencevalues, may interfere with the control in a certain manner. This is whythe temperature sensor 28 is preferably calibrated, e.g. by calibrationresistances, via which the influence of certain interference factors onthe control can be eliminated or reduced, ensuring a precise control ofthe heating output of the torch 7.

What is claimed is:
 1. A method of detecting the wear of the anode of awater-vapor plasma torch (7), said anode being designed as a nozzle (26)with an outlet opening (27), wherein a gas (20) escapes through theoutlet opening (27) formed by evaporation of a liquid (9), fed via afeed line (8), by means of a heating device (22), and an evaporator(23), wherein the nozzle (26) is worn out by enlarging the outletopening (27), thus increasing the flow rate of the liquid (9), whereinthe flow rate of the liquid (9) is measured in the feed line (8), andwherein the change in the flow rate is consulted as a measure for thewear of the nozzle (26).
 2. The wear-detection method according to claim1, wherein the flow rate is compared with a threshold value of the flowrate, which threshold value preferably ranges 60% above the flow rate ofa new nozzle, and wherein a change of the nozzle (26) is requested whenthe threshold value is reached.
 3. The wear-detection method accordingto claim 1, wherein the wear of the nozzle (26) is displayed on adisplay.
 4. The wear-detection method according to claim 1, wherein theflow rate detected is compared to the flow-rate reference valuesdeposited in the control unit (4).
 5. The wear-detection methodaccording to claim 4, wherein the flow-rate reference values aredeposited as a function of current strength, and wherein the currentstrength is taken into consideration when assessing wear of the nozzle(26).
 6. A method of detecting the wear of the anode of a water-vaporplasma torch (7), said anode being designed as a nozzle (26) with anoutlet opening (27), wherein a gas (20) escapes through the outletopening (27) formed by evaporation of a liquid (9), fed via a feed line(8), by means of a heating device (22), and an evaporator (23), whereinthe nozzle (26) is worn out by enlarging the outlet opening (27), thusincreasing the flow rate of the liquid (9), wherein the output of theheating element (22) is detected, and wherein the heating output whichis a function of the flow rate of the liquid (9) is consulted whenassessing wear of the nozzle (26).
 7. The wear-detection methodaccording to claim 6, wherein the heating output is determined via theelectric power fed to the heating device (22).
 8. The wear-detectionmethod according to claim 6, wherein the heating output of the heatingdevice (22) is controlled as a function of at least one temperature. 9.The wear-detection method according to claim 8, wherein the heatingoutput of the heating device (22) is controlled as a function of thetemperature within a piston rod (25), and the temperature within theevaporator (23).
 10. The wear-detection method according to claim 6,wherein the wear of the nozzle (26) is displayed on a display.
 11. Thewear-detection method according to claim 6, wherein the heating outputof the heating device (22) detected is compared to the heating-outputreference values deposited in the control unit (4).
 12. Thewear-detection method according to claim 6, wherein the heating-outputreference values are deposited as a function of the current strength,and wherein the current strength is taken into consideration whenassessing wear of the nozzle (26)
 13. A method of controlling a cuttingprocess conducted by means of a water-vapor plasma torch (7), wherein aliquid (9) is fed to the torch (7) via a feed line (8), and wherein theliquid (9) is evaporated into a gas (20) by means of a controlledheating device (22) and an evaporator (23), wherein the temperaturewithin a piston rod (25) of the torch (7) is detected by at least onetemperature sensor (28) arranged within the piston rod (25), and whereinthe heating device (22) is controlled as a function of the temperaturewithin the piston rod (25) in a manner to ensure the completeevaporation of the liquid (9) fed via the feed line (8).
 14. Theprocess-control method according to claim 13, wherein the temperaturevalues detected are processed by the control unit (4) which is connectedto the heating device (22), the control unit (4) thus controlling theheating device (22).
 15. The process-control method according to claim13, wherein the heating device (22) is controlled as a function of thetemperature values detected, taking into consideration the referencevalues deposited in the control unit (4).
 16. The process-control methodaccording to claim 13, wherein the output of the heating device (22) iscorrespondingly increased when the temperature values are decreasing.17. The process-control method according to claim 13, wherein the outputof the heating device (22) is correspondingly decreased when thetemperature values are increasing.
 18. The process-control methodaccording to claim 13, wherein the heating device (22) is controlledonly when an appropriate signal is sent by a protective-cap monitoringmeans.
 19. The process-control method according to claim 13, whereinalso a power element is started by the control unit (4) substantially atthe same time the control process is started.